Improve types

[knightedcodemonkey]

• Update should use an xor type in regards to code and error, i.e. only one should exist and be required.
• The error property of UpdateError should be renamed to isError to avoid error.error chains when accessing the error property of an Update.
• Consider switching interface to type.

Perhaps the first two bullet items should be resolved by making Update be

type Update = {
  codeOrError: string | UpdateError
  map?: SourceMap | null
}